The Early 65 Rally Pac might get in the way of your 66 gagues, even if you remove the eyebrows and center piece. I think the switch to the 5-gague cluster in 66 is why Rally Pacs were modified. I have a 6-cyl early 65 Rally Pac in my 65 convertible (sweep speedo and idiot lights). If the owner agrees, you might want to borrow it and try holding it on the steering column to see how it looks. I had mine apart while restoring it (converted 8-cyl tach for my 6 & upgraded the clock to quartz) and I doubt if you will like how it looks without the center piece and eyebrows. I know they make new repro Rally Pacs for 6-cyl Mustangs, but they are expensive. It's too bad if it's an original 6-cyl tach, as they are pretty rare. As to the wiring, All the Mustang vendors that sell the under hood wiring kit say it will work for both 65 and 66 Rally Pacs. I've attached a neat little write-up on Rally Pacs I found on the net a while back. I actually communicated with this guy when I was restoring mine and he knows his stuff. At least it will give you a better idea of the physical differences. Every now and then you will see a good buy on ebay for an 8-cyl Rally Pac. If you can get a good buy on an old 8-cyl, Rally Pac, then it might be worthwhile to send the tach to Gary the Tachman or someone else to convert it to run on a 6.
Good Luck!
__________________ 1965 Convertible 200 I-6, C-4 Auto (originally 3-speed manual) "Groovy Red" (originally Wimbleton White, then dark green) Newly restored and finally home!
1969 Mach 1 351 (W) 3-Speed Manual, Winter Blue, Deluxe Interior, All original, per Marti Report/Eminger Invoice. Finally moving her from the barn to the garage to begin work! | 
